This invention relates to structures and methods formed from high temperature superconductors. More particularly, it relates to devices such as resonators having high Q and reduced intermodulation distortion for use as passive microwave devices.
Electrical components come in various conventional forms, such as inductors, capacitors and resistors. A lumped electrical element is one whose physical size is substantially less than the wave length of the electro-magnetic field passing through the element. A distributed element is one whose size is larger than that for a lumped element. As an example, a lumped element in the form of an inductor would have a physical size which is a relatively small fraction of the wave length used with the circuit, typically less than xe2x85x9 of the wavelength.
Inductors, capacitors and resistors have been grouped together into useful circuits. Useful circuits including those elements include resonant circuits and filters. One particular application has been the formation of filters useful in the microwave range, such as above 500 MHZ.
Considering the case of conventional microwave filters, there have been basically three types. First, lumped element filters have used separately fabricated air wound inductors and parallel plate capacitors, wired together into a filter circuit. These conventional components are relatively small compared to the wave length, and accordingly, make for a fairly compact filters. However, the use of separate elements has proved to be difficult in manufacture, and resulting in large circuit to circuit differences. The second conventional filter structure utilizes mechanical distributed element components. Coupled bars or rods are used to form transmission line networks which are arranged as a filter circuit. Ordinarily, the length of the bars or rods is xc2xc or xc2xd of the wave length at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. The structures also suffer from various responses at multiples of the center frequency.
Various thin-filmed lumped element structures have been proposed. Swanson U.S. Pat. No. 4,881,050, issued Nov. 14, 1989, discloses a thin-film microwave filter utilizing lumped elements. In particular, a capacitor xcfx80 network utilizing spiral inductors and capacitors is disclosed. Generally, a multi-layer structure is utilized, a dielectric substrate having a ground plane on one side of the substrate and multiple thin-filmed metal layers and insulators on the other side. Filters are formed by configuring the metal and insulation layers to form capacitive xcfx80-networks and spiral inductors. Swanson U.S. Pat. No. 5,175,518 entitled xe2x80x9cWide Percentage Band With Microwave Filter Network and Method of Manufacturing Samexe2x80x9d discloses a lumped element thin-film based structure. Specifically, an alumina substrate has a ground plane on one side and multiple layer plate-like structures on the other side. A silicon nitride dielectric layer is deposited over the first plate on the substrate, and a second and third capacitor plates are deposited on the dielectric over the first plate.
Historically, such lumped element circuits were fabricated using normal, that is, non-superconducting materials. These materials have an inherent loss, and as a result, the circuits have various degree of lossiness. For resonant circuits, the loss is particularly critical. The Q of a device is a measure of its power dissipation or lossiness. Resonant circuits fabricated from normal metals have Q""s at best on the order of a few hundred.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from these materials. The microwave properties of the high temperature superconductors has improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. B. Hammond, et al., xe2x80x9cEpitaxial Tl2Ca1Ba2Cu2O8Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77 Kxe2x80x9d, Appl. Phy. Lett., Vol. 57, pp. 825-827, 1990. Various filter structures and resonators have been formed. Other discrete circuits for filters in the microwave region have been described. See, e.g., S. H. Talisa, et al., xe2x80x9cLow-and High-Temperature Superconducting Microwave Filters,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, Sep. 1991, pp. 1448-1554.
The need for compact, reliable narrow band filters has never been stronger. Applications in the telecommunication fields are of particular importance. As more users desire to use the microwave band, the use of narrow band filters will increase the number of users in the spectrum. The area from 800 to 2,000 MHZ is of particular interest. In the United States, the 800 to 900 MHz range is used for analog cellular communications. The personal communications services are planned for the 1,800 to 2,000 MHZ range.
Many passive microwave devices, for example, resonators, filters, antennas, delay lines and inductors, have been fabricated in planar form utilizing high temperature superconducting thin films. As described, such structures are often smaller than conventional technologies in terms of physical size. However, these devices are also limited in their size given the constraints of fabricating high quality, epitaxial films. As a result, devices fabricated in HTS films are often of a quasi-lumped element nature, that is, where the nominal size the device is smaller than the wavelength of operation. This often results in folding of devices, which leads to significant coupling between lines.
Despite the clear desirability of improved electrical circuits, including the known desirability of converting circuitry to include superconducting elements, efforts to date have been less than satisfactory in all regards. It has proved to be especially difficult in substituting high temperature superconducting materials to form circuits without severely degrading the intrinsic Q of the superconducting film. These problems include circuit structure, radiative loss and tuning and have remained in spite of the clear desirability of an improved circuit.
This patent relates to various novel structures and methods for forming high temperature superconducting devices, most particularly resonators. These devices have high Q, that is, at least in excess of 1,000, more preferably in excess of 25,000, and most preferably in excess of 50,000. Generally, these inventive structures reduce peak current densities relative to known structures. One significant result of reduced current density in reduced intermodulation effects.
In one aspect of this invention, a spiral snake resonator having a terminal end disposed within the resonator is provided. In the preferred mode of this embodiment, multiple long runs are connected by turns, where the turns at one end of the resonator are concentric semicircles, with the center of radius being disposed between long runs. The turns at the second ends of the resonator are also concentric semicircles, though with the center of curvature being disposed at the end of a centrally disposed long run.
In yet another aspect of this invention, a resonator having a relatively larger width of a long run where the current is higher is disclosed. Such a wide in the middle structure may be utilized in conjunction with resonators, especially a spiral in, spiral out resonator. Similarly, resonators may be constructed having varying widths of long runs where the width of the long runs is adapted to reduce the current density in those long runs which, if narrower, would have relatively higher current densities. Other factors, such as impedance matching, may be considered. In the preferred embodiment of this mode, the relative width of adjacent long runs in a zig-zag or snake resonator is 2:3.
In yet another aspect of this invention, resonators are operated in higher modes, that is modes above their fundamental mode. Such operation serves to reduce peak current densities, and also to reduce intermodulation effects.
In yet another aspect of this invention, it has been discovered that operation of a resonator in a symmetric mode provides improved results. Lower current densities are achieved due to currents flowing in substantially the same direction in adjacent legs of the resonator. Devices operated in the symmetric mode are particularly advantageous when for field effects are not significant or are otherwise compensated for.
In yet another aspect of this invention, a hairpin resonator structure operated in a harmonic mode is disclosed. The harmonic hairpin structure and mode results in a symmetric current flow, that is, current flow in the same direction in adjacent long runs within a group. Superior performance is obtained from such structures, resulting in a 15-20 dB improvement in intermodulation relative to operation of the hairpin at the fundamental frequency.
Accordingly, it is an object of this invention to provide improved high temperature superconducting structures.
It is yet a further object of this invention to provide improved resonators having reduced intermodulation.
It is yet a further object of this invention to provide resonators having reduced peak current densities.
It is yet a further object of this invention to provide high Q, superconducting resonators having reduced intermodulation effects.